1. Field of the Invention
This invention relates to focus adjustment of an optical system. More particularly, it relates to focus adjustment of a projection optical system for projecting a semiconductor integrated circuit pattern, formed on a mask or a reticle, onto a wafer provided with a photosensitive medium.
2. Description of the Prior Art
To adjust the focus position of a projection optical system, a method of adjusting the distance between an object plane or an image plane and a lens has heretofore often been employed. However, the method in which the object plane or the image plane is moved requires a large spatial margin for distance adjustment, and the method in which the lens is moved involves a great disadvantage because, during its movement, the lens may vibrate causing optical axis displacement which aggravates the quality of the image.
On the other hand, it is known to pour a liquid into a region between two membranes to form a lens and vary the power of the lens by increasing or decreasing the amount of the liquid poured. However, this type of lens has great aberrations and cannot be used for the projection of precise patterns.